This invention is concerned with improvements in and relating to adhesion promoters, especially for use in promoting the adhesion of addition polymerizable dental materials to natural teeth.
Addition polymerizable dental materials, such as filling materials, cements and the like, generally comprise a mixture of a finely divided filler with one or more ethylenically unsaturated materials capable of in situ polymerization in a dental cavity to form a hard filling material. A wide variety of fillers may be employed and examples thereof include finely divided glasses, ceramics and inorganic oxides contining e.g. silicon and aluminum. The filler will generally have been subjected to an appropriate surface treatment, such as silanation or with a titanate. The finely divided inert filler may form less than 50% by weight of the composite material, but commonly forms from 50 to 85% by weight thereof and especially from 60 to 80% by weight thereof. The inert filler suitably has an average particle size of from 1 to 100, preferably from 1 to 60 microns. Such a filler may be replaced by, or used in admixture with a superfine filler, e.g. having a particle size of from 10 to 400 millimicrons as disclosed in British Patent Specification No. 1,488,403, in which case the filling material may contain lower amount, e.g. 30-50% by weight, of filler.
Other addition polymerizable dental materials comprise a mixture of an organic filler, e.g. a polymer or copolymer of methyl methacrylate, together with an ethylenically unsaturated material, typically methyl methacrylate.
The ethylenically unsaturated component or binder of the composite material generally comprises one or more esters of ethylenically unsaturated carboxylic acids, especially of acryl or methacrylic acid. Thus, the ethylenically unsaturated binder component may comprise simple esters of unsaturated carboxylic acids with monohydric aliphatic alcohols, for example methyl (meth) acrylate, ethyl (meth)acrylate and 2-ethylhexyl (meth) acrylate.
(In this specification the term "(meth)acrylate" is intended to refer to an acrylic or methacrylic acid ester of a hydroxyl group-containing compound.)
Further, the unsaturated component may comprise unsaturated carboxylic esters of polyhydric alcohol, especially polyhydric alcohols containing one or more phenyl groups, such as the adduct of bisphenol A and glycidyl methacrylate, Bisphenol A (meth)acrylate and/or reaction products of 2-hydroxyethyl (meth) acrylate with isophthalic, terephthalic or phthalic acid chloride. Another class of ethylenically unsaturated binder the adducts of 2,2-propane-bis[3-4-phenoxy)-1,2-dihydroxx-propane-1-methacrylate] and an isocyanate or diisocyanate as described in U.S. Pat. No. 3,629,187. Yet another class of ethylenically unsaturated material which may be present in the ethylenically unsaturated binder component comprises the reaction products of monoesters of dihydric alcohols (e.g. ethylene glycol mono(meth)acrylate) with polyisocyanates, especially aliphatic diisocyanates, such as 2,2,4-trimethyl-hexamethylene diisocyanate or hexamethylene diisocyanate. In practice, a mixture of two or more ethylenically unsaturated materials may be, and commonly is, employed.
The unsaturated component may also comprise an unsaturated fluorine-containing material, as disclosed, for example, in U.S. Pat. No. 4,292,029, which additionally serves to reduce penetration of fluids between tooth surfaces and restorative material.
The composite materials should also contain, when filled into a tooth cavity, a polymerization initiator for effecting polymerization of the ethylenically unsaturated binder component. Such initiator should, of course, be capable of initiating polymerization at comparatively low temperatures, i.e. mouth temperatures, and examples of suitable initiators include organic peroxides, such as benzoyl peroxide or tert.butyl peroctanoate, desirably together with an activator, e.g. a tertiary amine activator, such as dimethyl paratoluidine, or ascorbic or sulphinic acid or a salt thereof. In an alternative procedure, the composite filling material may contain an initiator sensitive to visible or ultraviolet light which, on exposure to such light, initiates polymerization of the ethylenically unsaturated binder component. Examples of such initiators include chlorobenzophenone and benzoin methyl ether (for U.V. light) and camphorquinone (for visible light). In some cases, an accelerator, such as a secondary or tertiary amine, may also be employed in order to accelerate the reaction.
It is most desirable, when filling a tooth cavity with a composite filling material, as a dental restorative, to ensure good adhesion between the enamel on the tooth surrounding the cavity and the set (polymerized) composite material since there is thereby obtained a good seal between the set composite material and the enamel of the tooth which prevents, or at least markedly inhibits, ingress of mouth fluids into the filled cavity and so prevents further decay or loss of the filling. In order to achieve good adhesion between the filling material and the tooth enamel, it has been recommended to subject the enamel to an acid etch, for example, with 30-50% aqueous phosphoric acid or 55% aqueous orthophosphoric acid buffered with zinc oxide. This procedure, however, is time-consuming and, naturally, involves the loss or removal of some tooth enamel.